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Old 18th November 2008, 05:45 PM   #41
TimS is offline TimS  New Zealand
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Homemoder, Jam

The JLH buffer looks looks remarkable like a unity gain F5 with bipolar output stage.

I think there are several advantages to have a dual/split feedback:
- If the output clips at one of the rails, a single feedback amplifier would lose control of its feedback loop and it may oscillate or latch-up. Whereas if there is a second feedback loop, different voltage than the first, It would still be in control and your amplifier will be perfectly stable while clipping.
- If it is a symetrical complemetary design, like the F5 and the JLH, it can help to balance the 2 halves. ie instead of having 2 high gain stages (that are a complementary mirror to each other) in parallel, you put a feedback loop around each gain stage and then put them in parallel. You can even tweak the feedback of each half to ensure matching.

Jam,
Thanks for the feedback on my layout, I was hoping to keep cost down and just make it a 2 layer design and keep the ground plane on the top layer, but it looks like that is not possible.
Also, I like the FET/bipolar designs I will be giving them a closer look.
I am planning use my design to replace a pre-amp and I am thinking I will need another stage before the volume control so the FET/bipolar circuits may be a candidate.

Cheers

Tim
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Old 18th November 2008, 07:48 PM   #42
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Thanks Tim, your analasys might be correct, I never really looked that closely at the circuit. Its good to have input from others I never thought of looking at it that way. Jam also locked on to this quickly. Maybe spice can reveal something on the clipping performance compared to the other circuit. JLH didnt mention anything about this but then again reading his stuff one has to do a lot of maths to discover why he designed certain circuits the way he did. Yes the resemblance to F5 is remarkable.

As an output buffer for low impedance headphones I think I would also go with a diamond buffer, I havent tried the fet one, cant get japanese p channels here and the n parts are just as difficult.

Nelson its a difficult decision to make, sometimes I prefer one, other times the other. Using the fet bjt cfp in a ltp could have interesting results I agree. Another good sounding combination is bjt mosfet cfp, but Ive only tried it in outputstages.

Jam I think you sent me an email, sorry I have not responded yet, as I havent had access to my account for some days. I recieve some sensitive business related mail through this account after changing from others but someone has again been able to locate this public address of mine and gain access into it. I have the authoristies looking into this again so I cannot respond for the time being.
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Old 18th November 2008, 11:08 PM   #43
jam is offline jam  United States
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nelson,

I agree with you completely, increasing the bias on the fets usually helps remove some of the grain. Thanks for the feedback (no pun intended ).

Regards,

Jam
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Old 18th November 2008, 11:19 PM   #44
jam is offline jam  United States
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homemodder,

Sorry, Iwas under the impression that JLH used a single feedback path. It should sound very good as you say.
I wonder how it would compare to the jfet diamond buffer (non-feedback). This might be a good comparison between feedback and non-feedback circuits (I should say non-loop feedback before I get into trouble with the definition of feedback).

Regards,

Jam
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Old 18th November 2008, 11:30 PM   #45
jam is offline jam  United States
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Tim,

Your argument on split feedback makes sense. Seems to be the part of the reason why CFP output stages in amps have so many problems.

Regards,

Jam
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Old 20th November 2008, 04:54 AM   #46
lineup is offline lineup  Sweden
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Below I have attach ACD (from Denmark )
version of his Diamond Buffer.
It is used within op-amp loop, to provide a Class A current output.

As can be seen, he uses the drop of one 2SC2240 and one 2SA970
to set the voltage across the output emitter Resistors.
And so bias into true Class A.

He also sells his PCB (without components) for a fair price.
I judge this as a very good circuit when we need a powerful line level buffer with hi-Fi quality.
http://audio-innovation.eu/shop/prod...products_id=31

Lineup
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File Type: png acd_class-a_buffer.png (6.6 KB, 1067 views)
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Old 20th November 2008, 05:28 AM   #47
anatech is offline anatech  Canada
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Hi lineup,
Odd output devices he uses. Those are high voltage transistors and I would think that BD139 and BD140 would be better suited. There are some wonderful Japanese transistors that would work well there too.

-Chris
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Old 20th November 2008, 06:40 AM   #48
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What is the purpose of R10/11 resistors? If they are there for protection of output BJTs, I can't see how that would work? Also, is it a good idea to put diode connected Q3/4 for biasing output BJTs as this is another nonlinear element in circuit, so should we stick to simple resistors instead?
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Old 20th November 2008, 07:58 AM   #49
h_a is offline h_a  Europe
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You want temperature tracking for a stable bias. The resistor can't achieve that.

Have fun, Hannes
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Old 20th November 2008, 08:01 AM   #50
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Quote:
Originally posted by aparatusonitus
What is the purpose of R10/11 resistors? If they are there for protection of output BJTs, I can't see how that would work? Also, is it a good idea to put diode connected Q3/4 for biasing output BJTs as this is another nonlinear element in circuit, so should we stick to simple resistors instead?

The diode-transistor is current operated. Current operated diodes are linear (Iin = Iout).

Jan Didden
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